Geol 404 1st Edition Lecture 22Outline of Last Lecture I. Clastic ReservoirsII. 4th Order HeterogeneityIII. Depositional TextureIV. CompactionV. Pressure SolutionVI. Porosity TypesOutline of Current Lecture VII. Decision MakingVIII. Distribution of Prospect ReservoirsIX. Three Point MethodX. Log NormalityXI. Depth Structure MapCurrent LectureVII. Decision Makinga. The question is – who gets an allocation of this drilling budget money and most importantly, is that exploration opportunity the best one with the best chance of succeeding?i. Build a “prospect portfolio” based on hydrocarbon potential + Economics + Risk = Decisionii. The allocation of drilling money can then go to the best prospects in the portfolio while those prospects at the bottom of the portfolio can either be “nixed”, farmed-out or have more “G and G” money spent on identifying or reducing some of the existing risk elementsb. Probability of Occurrencei. The probability scale1. Use it subjectively, the value is that it conveys a uniform meaning. 2. The use of probability allows us to perform prospect descriptions and risk calculations that we cannot with mere words such as “good”, “poor”, “excellent”, “maximum”, “minimum” and “most-likely”.3. Zero means the event wont happen4. 1.0 point indicates a probability of 100% or certaintyc. Need For ConsistencyThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.i. While having a probability scale is a good start, there is still a modicum of inconsistency as to what the scaling quantifies between “zero” (won’t happen) to “1.0” (will happen)ii. One way of fixing the problem is to have a more rigid set of definitions applicable to each of the risks elements being quantifiedd. Geological Risk Elementsi. “five point” risk matrix of:- 1. – Source, seal, structure, reservoir and timing (e.g. Generation and migration). 2. The merits of this are that these are all risk elements needed in the construction of a critical elements chart. 3. Some companies however use four elements and describe them as – Psource – Preservoir – Ptrap – Pdynamicsii. Three risk elements:- 1. – Structure – Reservoir – Environment (including paleohistory, timing aspects, proximity of source beds, shows, etc)VIII. Distribution of Prospect Reservoirsa. First we must look at how a simple volumetric reserve is calculated. b. A prospect reserve calculation can be expressed by simply multiplying the “productive area” (in acres) with the “average net pay” (in feet) and with a “hydrocarbon-recovery factor” (in bbl or mcf per net acre-foot) c. This is a very deterministic; that is, single-value estimates for each parameter, all of which, because of substantial geotechnical uncertainty could be much better forecast as a probabilistic range of possible outcomes.IX. Three Point Methoda. The method uses as input a calculated range of values for each parameter of “Area”, “Net pay” and “HC-Recovery” such that these values corresponding to 10%, 50% and 90% probability of occurrence. b. For an exercise we’ll take a hypothetical prospect and follow the calculation as a series ofsteps looking at “Area”, “Average Net Pay” and “HC-Recovery Factor”X. Log Normalitya. First though, we have to have a means of plotting this probability data and for this we use “Cumulative log probability Graph” paper (or equivalent computer program) b. The special property of a cumulative probability graph is that a cumulative probability distribution that is perfectly normal will plot as a straight, sloping line. XI. Depth Structure Mapa. For making accurate reserve predictions, the petroleum geologists first priority is to make a depth structure map on the reservoir/seal interface (or as close as he/she can make it).b. All faults, depositional/erosional edge lines, wells, shows and seeps should also be marked.c. Conceptually, this line represents the full universe of all possible outcomes – 100% - and probability is expressed as a cumulative percent of some outcome “equal to or less than” or “equal to or more than” a particular value.d. Step 1. Let’s say our geologist has mapped a four-way closure (plunging anticline) on 2-D seismic data. i. The 2-D data set is reasonable but still has areas of concern, perhaps a spill pointor fault need better detailing but for the moment he is confident enough that bycontouring a closure up-dip of the spill point, he can planimeter an area of closure of 600 acres. ii. This value is taken to represent a P90% probability of occurrence; in other wordsthere is only a 10% chance the closure could be bigger. iii. This value is then plotted on our cumulative log probability graph which has “area” (in acres) on the “X” axis and cumulative probability of occurrence on the “Y” axis.e. Step 2. Looking through all the available e-logs from adjacent wells, our geologist thinks that some of the better wells show approximately 40’ of net pay which is consistent withhis depositional model (let’s say a braided stream environment). He calls this P90 realizing that there may be a 10% chance that some wells could show even more net paysand.f. Step 3. Hydrocarbon-recovery factor expresses “reservoir yield” as barrels (bbl) of oil or mcf (thousand cubic feet) of natural gas per acre-foot of